The element sub is defined in module text. It belongs to group format.
Content model(#PCDATA | em | u | strong | br | sup | tt | sub | pagenumber)*
These elements contain sub:
address, articletitle, caption, dean, degree, editor, em, foreign, freehead, glossref, head, indexref, keyword, legend, line, link, major, note, organization, p, pre, publisher, q, school, serialtitle, speaker, stage, strong, sub, submission, sup, term, title, tt, u, worktitle.
These elements may occur inside of sub:
#PCDATA, br, em, strong, sub, sup, tt, u